1. Field of the Invention
The present invention relates to a sheet processing device, an image forming system, and a method of additionally folding a sheet bundle, and specifically relates to a sheet processing device having a function for folding a sheet recording medium such as paper, recording paper, and transfer paper (hereinafter, simply referred to as a “sheet” in this specification), an image forming system including the sheet processing device, and a method of additionally folding a sheet bundle performed by the sheet processing device.
2. Description of the Related Art
Conventionally, in some postprocessing devices used in combination with an image forming apparatus such as a copying machine, center parts of one or more sheets are stitched and the center part of the sheet bundle is folded with a pair of folding rollers arranged in parallel in a sheet folding direction to bind a saddle-stitched booklet. Already known is a technique for additional-folding with a roller moving along a spine of the booklet to reinforce a fold of the saddle-stitched booklet.
In such an additional-folding technique, a roller waiting outside the booklet (sheet bundle) is moved on the spine (fold line part) of the booklet to additionally fold the spine of the booklet with an additional-folding roller.
As such kind of additional-folding technique, known is the invention disclosed in Japanese Patent Application Laid-open No. 2009-126685 or Japanese Patent Application Laid-open No. 2006-321622, for example.
Japanese Patent Application Laid-open No. 2009-126685 discloses a sheet folding device including a folding unit that folds a carried sheet-like recording medium, and an additional-folding unit that moves and pressurizes on a fold line part of the sheet recording medium in a direction orthogonal to a sheet carrying direction to perform additional-folding after the folding processing by the folding unit. In the sheet folding device, the additional-folding unit is arranged to be inclined in a direction in which a force is generated in a moving direction when the fold line part is pressurized with respect to a normal on a medium surface of the sheet recording medium.
Japanese Patent Application Laid-open No. 2006-321622 discloses a sheet bundle spine processing device including a fold processing unit that moves while pinching front and rear surfaces of a spine fold line part of a folded sheet bundle to arrange the shape of the fold line part, a spine processing unit that moves while pressing a spine of the spine fold line part of the sheet bundle to flatten the spine, and a selection unit that selects and activates at least one of the fold processing unit and the spine processing unit.
In the technique disclosed in Japanese Patent Application Laid-open No. 2009-126685, although a direction of an energizing force is inclined from a moving direction of an additional-folding mechanism in order to reduce a load, the additional-folding mechanism is configured by a fixed receiving member opposed to a pressure roller, so that a pressing force to a sheet bundle is generated in the thickness direction of the sheet bundle. Due to this, rigidity is required for the device, the size of the device is increased, and the cost is increased accordingly.
In the technique disclosed in Japanese Patent Application Laid-open No. 2006-321622, a fold processing unit for reinforcing a fold line part includes three or more additional-folding rollers. The fold processing unit including a pair of two rollers for reinforcing the fold line part generates a pressurizing force in a direction orthogonal to a moving direction, and the third roller for flattening the spine of the fold line part generates the pressurizing force in a direction orthogonal to the pressurizing force generated by the pair of two rollers in a sheet carrying direction. Due to this, similarly to the technique disclosed in Japanese Patent Application Laid-open No. 2009-126685, rigidity is required for the device, the size of the device is increased, and the cost is increased accordingly.
In view of the conventional arts, there is a need to enable the additional-folding with a small pressurizing force, and reduce the size and cost of the device.